1. Field of the Invention
The present invention relates generally to a patch antenna with a dielectric separated from a patch plane, more particularly, to a patch antenna with a dielectric plate separated from a patch plane by an air gap to increase the gain of the antenna for a millimeter wave frequency range from 30 to 300 GHz and microwave frequencies near the millimeter wave frequencies.
2. Description of the Related Art
A patch antenna is thin and compact in shape, so the antenna is used in millimeter wave radio communication. Note that in the present specification, a patch antenna is defined as an antenna including a patch plane provided with high frequency power for radiating radio waves and a ground plane separated from the patch plane, wherein the patch plane and the ground plane are generally formed on opposed surfaces of a dielectric substrate. Since in millimeter waves, patch antennas have low gain, improvement has been performed on the gain by use of an array configuration or a dielectric lens.
However, an array antenna has a plurality of patch planes arranged on a dielectric substrate and there is a necessity for supplying power to respective patch planes with controlling the values and phases thereof and in addition, for distributing the power supply through a micro strip line along which power transfer loss is comparatively large in millimeter waves; therefore it is not easy that an actual practice coincides with its design. Further, when a dielectric substance which is low in power transfer loss is selected, it results in increase in cost of the antenna. Furthermore, since it is necessary to dispose patch planes spaced apart from each other by a distance equal to or more than 0.5xcex to xcex, where xcex is a wavelength, the area of an array antenna is large.
Whereas in order to improve the gain of a patch antenna using a dielectric lens, it is necessary for a lens to be larger than the angular aperture of the patch antenna, and on the other hand, since this angular aperture is generally wide, a large lens is necessary. Moreover, in order to obtain a high efficiency antenna, alignment precision between the patch antenna and the dielectric lens has to be high, which in turn requires high levels of techniques associated with assembly and inspection, leading to high cost.
In order to solve such problems with using a patch antenna, there is disclosed in JP 6-809715 A an antenna as shown in FIG. 10.
A patch antenna 10 is disposed between a reflection plate 11 and a dielectric block 12 with spacing from the reflection plate 11. A spacer 13 is placed between the reflection plate 11 and the dielectric block 12 and a micro strip line 14 is connected to the patch plane of the patch antenna 10.
The publication discloses that a gain can be increased by making multiple reflections, between the reflection plate 11 and the dielectric block 12, of radio waves radiated from the patch antenna 10 and aligning the phase planes of radio waves transmitted through the dielectric block 12 so as to increase the directivity of the antenna, and further by resonating the radio waves in the dielectric block.
In the antenna of FIG. 10, however, not only the dielectric block 12 but also the reflection plate 11 has to be added to the patch antenna 10, and moreover it is necessary to optimize a distance between the patch antenna 10 and the dielectric block 12, a thickness of the dielectric block 12, and further a distance between the patch antenna 10 and the reflection plate 11.
Accordingly, it is an object of the present invention to provide an improved patch antenna capable of increasing the gain with simpler configuration.
In a first aspect of an antenna according to the present invention, a dielectric member is disposed on the patch plane side of an patch antenna opposite to the patch plane with a distance of 0.1xcex0 to 2xcex0 from the patch plane, where xcex0 denotes a wavelength of a radio wave, in a free space, radiated from the antenna. A plane located opposite to the patch antenna on the opposite side to the dielectric member with respect to the patch antenna may be either a non-conductive plane or a conductive plane. In the case of the conductive plane, it is not necessary to adjust distances among the patch antenna, the dielectric member and the conductive plane so as to make phases of radiated radio wave coincident as in the above described prior art configuration. The conductive plane is separated from the dielectric member by such a distance that phases of the radio wave directly reached an incident surface of the dielectric member are substantially different from those indirectly reached the incident surface after having been reflected by the conductive plane.
According to the antenna of the present invention, by providing high frequency power to the patch antenna, a radio wave is radiated from the patch plane and passes through the dielectric member. The dielectric member is polarized by the electromagnetic wave and electromagnetic field is provided to the patch plane from the dielectric member to change the current distribution in the patch plane. By determining the distance between the dielectric member and the patch plane as described above, he current density grows larger mainly at a peripheral portion of the patch plane compared with a case where no dielectric member is employed. Thereby directivity arises in electromagnetic radiation pattern to increase the gain. A current distribution on the patch plane is controlled such that the directivity arises in the electromagnetic radiation pattern to increase the gain by operation of the dielectric member.
The principle of the present invention for achieving high gain is different from that of the known configuration employing the reflection plate 11 as shown in FIG. 10, and there is no need to employ the reflection plate 11 whose position is precisely adjusted; therefore the patch antenna of the first embodiment can increase the gain with a simpler configuration. That is, in this known configuration, strict positioning of the reflection plate 11 and others is required in order to make phases coincident between a radio wave directly transmitted through the dielectric member after having been radiated from the patch antenna and radio waves indirectly transmitted through the dielectric member after having been reflected by the reflection plate 11, whereas the present invention requires no such positioning even when the conductive plane is provided. It is a unique conception of the present invention to achieve high gain of the antenna with increasing current densities at a peripheral portion of the patch plane by the dielectric member.
In order to realize the present invention, it is only required that a dielectric member is disposed on the patch plane side of the patch antenna opposite to the patch plane with a distance of 0.1xcex0 to 2xcex0 from the patch plane, and a plane located opposite to the patch antenna on the opposite side to the dielectric member with respect to the patch antenna may be a non-conductive plane, that is, a nonreflective plane. In a case where the plane is a conductive plane, it is separated from the patch antenna or the dielectric member by such a distance that phases of the radio wave directly reached an incident surface of the dielectric member are substantially different from those indirectly reached the incident surface after having been reflected by the conductive plane. In order to realize the substantially different phases, it may be performed that the phase of the radio wave directly reached the incident surface of the dielectric member is determined, the phase of the radio wave indirectly reached the incident surface after having been reflected by the conductive plane is determined, and the both phases are made substantially different from each other, for example, opposite to each other. In design of the antenna, it may be performed that simulation of radiation pattern of is performed with taking into consideration dielectric constants of respective portions of the antenna according to the present invention and phase shifts of radio waves passing through the respective portions, and the phase condition is derived from the results of the simulation.
In a second aspect of an antenna according to the present invention, the dielectric member has a thickness of from 0.1xcex to 2xcex in the first aspect, where xcex is a wavelength of the radiated radio wave in the dielectric member.
According to this antenna, the electromagnetic field provided to the patch plane from the dielectric member is strengthened compared with a case where the thicknesses fall outside this range, and thereby the above effect is enhanced.
In a third aspect of an antenna according to the present invention, the dielectric member has a first dielectric in a middle portion thereof and a second dielectric disposed around the middle portion with a dielectric constant lower than that of the first dielectric in the first aspect.
According to this antenna, since the dielectric member also works as a dielectric lens, a directivity is increased more than in the first aspect, thereby increasing the gain of the antenna.
In one aspect of a communication module according to the present invention, since the dielectric member is attached to the cover of the communication module, high gain of the antenna can be achieved with substantially the same size as a prior art patch antenna.